As Google+ gets better and better, e.g. with the addition of hashtags yesterday (you know you want #cake)*, after three months fairly intensive use, I'm still trying to grok the subtleties of Google+.
I have a nice Edu Circle, and I have a nice Science Circle. Some people are in both Circles. But most aren't. I have some other circles, such as a University circle where I share locality-related items. But increasingly, I find myself sharing to My Circles (all of them) or Public.
The longer I spend on Google+, the fewer Circles I want. Would my Edu people mind if I merged them with my Science people, and vice versa? Or, for general purposes, shall I put you all in one big Circle and use Google+ like Twitter?
* (Hashtags work differently to Twitter, e.g. on Google+, #cake returns a search for "cake" where the hash is ignored, so if you're integrating an event tag across both platforms (and you're going to have to), make sure the tag string is sufficiently unique to avoid noise from common usage).